This invention relates to a contact-type image scanner, and more particularly to the signal processing of contact-type CCD image sensors used in a contact-type image scanner.
The prior art in this field has been described in, for example, Imeji Sensa no Erabikata, Tsukaikata (Selecting and Using Image Sensors) by Mitsuo Oshima, Nikkan Kogyo Shimbunsha, January 25, 1985, pp. 41-49 and 198-200.
As shown in the above-cited reference, the basic function of this type of device was to transfer photoreceptor data, which are stored as electric charges in a set of photodiodes during the interval between a pair of transfer pulses .phi..sub.T, through a photogate and transfer gate to a CCD (Charge-Coupled Device) analog register, then out through a preamplifier which reshapes the data by means of a reset pulse .phi..sub.R.
Three CCD image sensor configurations have been used.
FIG. 1 through FIG. 6 are block diagrams and timing charts of these configurations.
In FIG. 1 and FIG. 2, the numeral 1 denotes an input terminal for a transfer pulse (.phi..sub.T), A is a signal line for .phi..sub.T, 2 is an input terminal for a clock pulse (.phi.), B is a signal line for .phi., 3 is an input terminal for a reset pulse (.phi..sub.R), C is a signal line for .phi..sub.R, D is a signal line for output data, and 4 is an output terminal for the output data.
In FIG. 1, the data received by the photodiodes 5 pass through a photogate 6, are time-limited by .phi..sub.T in a transfer gate 7, and are fed to the CCD analog register 8. Thus only the light data received during the interval from one .phi..sub.T to the next .phi..sub.T can charge the photodiodes 5 to produce photosensor outputs. The photosensor outputs are fed in parallel to the bits of the CCD analog register 8.
The data are output in serial form on the .phi. clock of the CCD analog register 8 to a preamplifier 9. In the preamplifier 9, data corresponding to each bit are sampled on .phi..sub.R and output from the output terminal 4. As shown in FIG. 2, the data are output on the falling edge of .phi. and reset on the rising edge of .phi..sub.R.
In FIG. 3 and FIG. 4, the photodiodes 5 are divided into an even-numbered row and an odd-numbered row, each of which feeds through a photogate 6, a transfer gate 7, and a CCD analog register 8 to an output gate 10. At the output gate 10, the outputs from each photosensor are multiplexed, then they are sampled by a preamplifier 9. In FIG. 3 and FIG. 4, 2.sub.O and 2.sub.E are input terminals for .phi. and .phi., and B.sub.O and B.sub.E are signal lines for .phi. and .phi..
The basic data transfer operation proceeds similarly to FIG. 1 and FIG. 2. This system is frequently used in CCD sensors with around 2K bits, because it reduces the transfer time by half as compared with the CCD sensor shown in FIG. 1 and FIG. 2.
In FIG. 5 and FIG. 6, as in FIG. 3 and FIG. 4, the photoreceptor data are divided into an even row and an odd row, but in this system there is no output gate; instead there are two outputs with independent preamplifiers 9. This system shortens the transfer time as in FIG. 3 and FIG. 4, and increases the pulse width of the output signal. In FIG. 5 and FIG. 6, 3.sub.O and 3.sub.E are input terminals for .phi..sub.R, C.sub.O and C.sub.E are signal lines for .phi..sub.R, D.sub.O and D.sub.E are output data, and 4.sub.O and 4.sub.E are output terminals for the output data.
The system in FIG. 1 and FIG. 2 will be called the single-output system, the system in FIG. 3 and FIG. 4 will be called the multiplex-output system, and the system in FIG. 5 and FIG. 6 will be called the dual-output system.
Image scanners using these CCD image sensors fall into two general types: the reducing type and the contact type. A reducing-type image scanner scans an image by means of one or more CCD image sensors with ultra-small photocells, which receive reflected light from a light source in amounts proportional to an image being scanned through a reducing optical system. A contact-type image scanner has no reducing optical system, so the scanning resolution is in one-to-one ratio to the resolution of the photocells.
Next, a contact-type image sensor to which the prior art systems as well as this invention applies will be described with reference to FIG. 7 and FIG. 8.
An LED light source 11 illuminates a line on a document 12. The light reflected by the image on the document is focused by a Selfoc lens array (SLA) 13 and received by the photocells of a CCD image sensor 14 disposed on a substrate 15. The main scanning size of each CCD image sensor is shorter than the size (e.g., width) of the document, so a row of several sensors are used. The driving system depends on which of the types of CCD image sensors shown in FIG. 1 through FIG. 6 is used.
In a contact-type image scanner having several of the CCD image sensors described above arranged in a row, however, regardless of which driving system was employed, independent driving of each sensor entailed a great deal of complex processing of the output signals, and operation was slow. These points were unsatisfactory from a technical standpoint.